MONTPELIER — The Environmental Protection Agency is working with the state of Vermont to determine if more Rutland County homes were contaminated with a banned pesticide that the state believes was used by a pest control company.

High levels of the chemical chlorpyrifos, banned since 2001, have forced six households to relocate; five others have been advised to move.

Vermont health officials think more homes may have been contaminated by the chemical that they believe was used to treat bed bugs, Vermont Public Radio reports. The chemical can cause nervous system and developmental disorders.

North Clarendon exterminator Cary Buck has denied using chlorpyrifos; he told authorities he hasn’t used it in eight or nine years. But officials said they’ve found overwhelming evidence that he has used the pesticide more recently. The Vermont Agency of Agriculture suspended his exterminator license in June.

Tom Condon of the Environmental Protection Agency’s Boston bureau said dozens of EPA field agents will help with the testing of homes and to set up a database on the cases.

“This is unusual for us in that under most situations we’re dealing with contamination in the environment. This is indoor contamination that puts it in a special and unusual category for us,” he said.

Health officials believe they have identified the worst contaminated residences and that no one was become sick, said Joanne Calvi, district director for the state Health Department’s Rutland office.

Finding temporary housing for the affected households has been a challenge, she said.